In the applicant's prior U.S. Pat. No. 5,185,490 a key structure is shown in FIG. 15. This key structure is intended mainly for a terraced keyboard, e.g., a Janko Keyboard. A modification of this structure is suggested in column 17, line 28. In both the shown structure and the suggested modification the two-bushing pin is in the rear and the underside of the key adjacent the two pins is the same elevation, i.e., it is the same distance from the key top. This arrangement has been found to be less than ideal for several reasons:
Wobble is greater than it can be since either the front pin is positioned to the rear of the guard flange (and closer to the rear pin) or the underside of the key adjacent the rear pin is extended downward to the same elevation as the bottom of the guard flange (thus, extending the rear bushing downward as well). To minimize wobble, it is desirable to place the two pins as far from each other as possible, and also to place the bushings as close to the key top as possible. These placements will also serve to minimize bushing friction and wear by minimizing the leverage with which side force placed on the key is transmitted to the bushings.
Also, placing the two-bushing pin at the rear has been found to be less than ideal. Stress analysis has shown that a greater cross-sectional area (diameter) is required for the two-bushing pin than for the one-bushing pin. (This difference in pin requirements is discussed further in the description of the preferred embodiment below.) To minimize the amount of material used to make the pins, the two-bushing pin will thus be thicker than the one-bushing pin. The one-bushing pin therefore requires less length embedded in the key than the two-bushing pin to guarantee against being pryed out of the key by extreme side force. Thus, in this respect, the prior art embodiment with no guard flange is preferred (since the two-bushing pin can be imbedded deeper into the key), but, as stated earlier, friction and wobble are increased in this embodiment.
Another difficulty found with the guard flange embodiment: In order to provide plenty of clearance between the guard flange and the front bushing, the front pin must be placed significantly to the rear of the guard flange. This places the two pins even closer together which, as stated earlier, is not desirable.
Also, reducing the design as shown to practice has proven to be more difficult than expected. Specifically, it is necessary that the keyboard may be easily disassembled for repair. It is also necessary that the frame holding the bushings is rigid so the keyboard has a solid feel. If the frame for all key rows were formed as a single aluminum extrusion, it could be made quite rigid. But this might make access to the various mechanisms (return spring, key motion sensors, bushings, etc.) difficult.
One solution is to place the return springs and motion sensors under the frame. However, this would increase the height of the overall key mechanism, which is already somewhat high due to the necessary length of the two-bushing guide pin.
The key rows may be mounted on discrete key frame rails, (extruded separately), but these rails would tend to be less rigid than a single extrusion. Large amounts of aluminum could be used in each extrusion to add rigidity, but this would also add cost and weight. Thus, another means must be found to reinforce individual rails.
The applicant conceived the idea of connecting the rails together so they could reinforce each other. However, with the two bushings of each key at the same elevation, the upper front bushings of one row are at a higher elevation than the rear bushings of the adjacent row in front. It has been found that this non-alignment of the adjacent bushings makes it very difficult to connect the rails together without sacrificing other desired qualities. The most practical way found by the applicant for providing discrete rails has required a departure from the prior art.